Liquid Nitrogen: Part 1

In the first of a series, I would like to share my experience experimenting with the cryogen, liquid nitrogen in my home kitchen.  But first some background. Cryogens are cold liquids, anything -150°C or less, and liquid nitrogen (LN2) which boils at -192°C is very cold!  (It freezes at -210°C; anything between these two temperatures it is liquid.)

Normally, nitrogen is a gas and is all around us. In fact, the bulk of the air we breathe is nitrogen; oxygen makes up only 21% with virtually all the rest nitrogen. By super cooling air, it can be liquified from which the nitrogen can be extracted in liquid form.

Surprisingly, although super cold, LN2 isn’t a particularly efficient coolant. This is in part because it boils immediately on contact with a warmer object creating a gaseous layer of nitrogen insulating the cold liquid from the object. This is known as the Leidenfrost effect and explains why brief contact with LN2 doesn’t freeze skin. 

 This phenomenon applies to any liquid in contact with an object significantly hotter than its boiling point. Likely you have encountered this if you have ever tested how hot your frying pan was by putting water in it. When really hot, the water forms into a drop and slides across the pan.

Demonstration and explanation of the Leidenfrost effect.

LN2 behaves the same way because it boils at -192°C. Relatively speaking, your skin temperature is very hot at 37°C and just like a hot frying pan boils the LN2. After a few seconds though, the object / skin cools enough that the LN2 no longer boils off and freezing occurs.  The team at Modernist Cuisine has filmed an amazing high-speed video showing the LN2 leidenfrost effect:

High-Speed Video: liquid nitrogen leidenfrost effect shot at 3000 frames per second

Aside from the obvious that it can freeze skin, another very important safety consideration is that when LN2 turns to gas, which it does at anything above its boiling point of -192°C, it expands. And it expands a lot! At room temperature it is a whopping 645 times its original volume. This creates phenomenal pressure if LN2 is kept in an enclosed container.  For this reason, it should only be stored in specialized containers known as Dewars.  These vacuum-insulated containers help maintain the contents' temperature for extended periods just like a Thermos® bottle. But unlike a sealed Thermos®, it has special release valves to prevent pressure build up.  The importance of this can’t be stressed enough. An infamous incident from January 2006 illustrates this danger.

At Texas A&M University, the pressure-relief devices of a tank of LN2 were malfunctioning and sealed to prevent loss of LN2. Because of the subsequent pressure buildup, the tank failed catastrophically. The explosion propelled the tank through the ceiling above, shattered a reinforced concrete beam immediately below it, and blew the walls of the laboratory off their foundations.  Take home message; never store LN2 in a sealed container.

What happens when LN2 is stored in a sealed container

There are other important safety considerations, but I will discuss these in future instalments.

Now to the original point of this blog, the culinary uses for LN2. There are many, but I will start with Cryofrying.  I had the opportunity to purchase a mixed quarter of Wagyu (stay tuned for Wagyu blog!). This provided some delicious prime cuts, but there was a LOT of burger.  Wagyu is bred to produce meat with lots of intramuscular fat. However all that marbling can cause flare ups when grilling. And because it is so expensive, you don’t want to risk overcooking it.

Another challenge in searing hamburger is that the surface is very irregular with all kinds of little crevices. One risks overcooking the ridges to reach the crevasses or undercooking the crevices so as not to overcook the ridges. So how to perfectly cook the inside but at same time create a beautiful seared crust? 

Nathan Myhrvold at Modernist Cuisine tackled the problem this way.  Cook the burger sous vide to a perfect medium rare, briefly dip the burger in LN2 to freeze a thin coating around the burger, and then deep fry the burger.  LN2 is so cold the quick dip freezes the outside before it has a chance to cool the inside.  The frozen outside protects the inside when deep frying so you don’t overcook the inside.  Because the hot oil is able to penetrate all the nooks and crannies, you end up with a beautiful caramelized crust with preservation of the perfectly cooked inside!

HOW TO CRYOFY HAMBURGER À LA MODERNIST CUISINE

1. Form loose patties (200gm / 3cm thick)

2. Place in sous vide or zip lock bag. Remove air by water displacement as vacuum packing would compress patty.

3. Cook sous vide (Temperature a personal choice; I prefer 56°C / 133°F)

Burgers cooked sous vide at 56°C

4. Remove burgers and pat dry.

Drying surface thoroughly

5. Dip in LN2 x 30seconds.

6. Deep fry in oil at 232°C / 450°F x 1min or until crispy brown exterior.

Obviously one isn’t going to prepare burgers like this as a matter of routine. However it does illustrates how one can use LN2 in the kitchen to produce some spectacular results. Stay tuned for some other culinary uses of LN2!